Scandal A: A prominent politician gets caught sleeping with a campaign aide and plunges himself into an ugly paternity dispute — all while his cancer-stricken wife is fighting for her life.

Scandal B: A prominent politician’s signature health-care plan turns out to have been put together badly, and he is forced to confess that the plan will cost taxpayers billions more than expected.

It’s a no-brainer which scandal is likely to catch — and keep — our attention. The interesting question as the presidential election heads into the homestretch is why we care more about some stories that do not affect us directly, even as we tune out other stories that do.

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. . . . Why are we more likely to discuss a gossipy rumor at a party than a policy error that can actually make a material difference to our own lives?

One explanation is that cultural mores attune us to certain stories — we live in an era where gossipy scandals rule. To test this, psychologist Hank Davis at the University of Guelph in Ontario examined hundreds of sensational stories on the front pages of newspapers in eight countries over a 300-year period, from 1701 to 2001.

Remarkably, he concluded that the themes of sensational news were identical not only across the centuries but also in diverse geographic locales — from the United States to Bangladesh, from Canada to Mauritius. . . .

The stories were sometimes about important things and sometimes not, but they nearly always involved the kind of themes that people who are part of small groups like to know about one another: lying and cheating, altruism and heroism, loyalty and disloyalty.

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Davis and other evolutionary psychologists argue that the reason John Edwards’s adultery has more zing in our heads than a dry policy dispute that could cost taxpayers billions of dollars is that the human brain evolved in a period where there were significant survival advantages to finding out the secrets of others. Since humans lived in small groups, the things you learned about other people’s character could tell you whom to trust when you were in a tight spot.

“We are continuing to navigate through the modern world with a Stone Age mind,” Davis said.

In the Pleistocene era, he added, there was no survival value in being able to decipher a health-care initiative, but there was significant value in information about “who needs a favor, who is in a position to offer one, who is trustworthy, who is a liar, who is available sexually, who is under the protection of a jealous partner, who is likely to abandon a family, who poses a threat to us.”

We may consciously know that we are no longer living in small hunter-gatherer groups and that it no longer makes sense to evaluate someone like Edwards as we might a friend or intimate partner, but our reptilian brain doesn’t realize this. Our prefrontal cortex might reason that a man who cheats on his wife while she is fighting cancer could make a perfectly fine president in a complex world, but the visceral distaste people feel about Edwards stems from there being an ancient part of the human brain that says, “Gee, I don’t want to get mixed up with this guy, because even in my hour of greatest need I might not be able to count on him,” said Frank T. McAndrew, an evolutionary social psychologist at Knox College in Illinois.

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“The human brain does not have any special module for evaluating welfare policy or immigration policy, but it has modules for evaluating people on the basis of character,” said Satoshi Kanazawa, an evolutionary psychologist at the London School of Economics. “That is probably why we have this gut reaction to affairs and marriages and lying. All of those things existed in the ancestral environment 100,000 years ago.”